Muscles generated through aerobic exercise consume body fats by using fats rather than carbohydrates as primary energy source. Since less carbohydrate is consumed, the blood sugar level is maintained and less hunger is experienced. In addition, since ATP is produced from fats rather than from carbohydrates, less reactive oxygen species are produced during the ATP generation and cell damage by the reactive oxygen species can be prevented. However, not all exercise exhibits such effect. Only aerobic exercises such as running and bicycling can provide such effect since they change the fast-twitch type muscles into slow-twitch type.
Human muscle consists of fast-twitch (type II) muscles and slow-twitch (type I) muscles approximately half-and-half. The fast-twitch muscle looks white since it lacks myoglobin and uses carbohydrates as primary energy source. This type of muscle is mainly formed through anaerobic exercise such as weight training. It has large muscle mass and is used for quick motion. In contrast, the slow-twitch muscle looks red because of myoglobin and uses fats instead of carbohydrates as primary energy source. It is mainly formed through aerobic exercise, has small muscle mass and is used for slow, long motion. The slow-twitch muscles enable exercise for a long time since they are rich in mitochondria and have high mitochondrial activity and thus resistance to fatigue is improved.
In general, sprinting athletes have fast-twitch muscles and slow-twitch muscles at a proportion of about 70:30 and have well-developed upper bodies. In contrast, marathoners usually have fast-twitch muscles and slow-twitch muscles at a proportion of about 30:70 and tend to have slim bodies. Marathoners are undersized than non-athletes of the same body weight (fast-twitch muscle:slow-twitch muscle=about 50:50) because muscles are about 5 times smaller in volume than fats of the same weight. To have a body of a marathoner through aerobic exercise, one should take exercise of 65-75% intensity based on the maximum heart rate for over 6 weeks, at least 3 times a week, more than 40 minutes a day, which is not so easy.